Oil and/or gas production in deep water may be carried out from facilities set on buoyant substructures. These substructures may comprise ship shaped Floating Production Storage and Offloading vessels (FPSO's), semisubmersibles, Tension Leg Platforms (TLP's) or spars. When using FPSO's or semisubmersibles for production, wellheads are usually located on the seabed. For both TLP's and spars, wellheads can be disposed above sea level.
The use of spars for drilling and production is a relatively recent development. (The Brent spar was merely for storage and offloading.) The Oryx Neptune spar (for production) was installed in August 1996. The design, fabrication and installation of that spar were described in OTC Papers 8384 and 8385. These papers were presented at the Offshore Technology Conference in Houston Tex. during May 1997. Subsequently, spars were designed for the Chevron Genesis (drilling and production) and Exxon Diana developments.
The spars referred to above were constructed primarily of steel. It has also been suggested that spars should be made of concrete. Spar configurations for construction in concrete were described on pages 29--33 of Offshore Engineer for April 1996.
In addition to the "spar" substructures described above, proposals have been made for other types of floating substructures designed generally on the spar principle.
One such proposal is set out in PCT Patent Specification No WO96/14473. This shows a single cylindrical hull, and a downward extension formed of four vertical legs of reduced diameter. The cylindrical hull is buoyant, and supports a deck. The vertical legs are connected together by diagonal truss members. The substructure is held in place by an array of semi taut mooring lines.
Our UK Patent Specification No 2,147,546A describes a multi column floating substructure. This has four corner columns which support a deck. The columns are buoyant, and are of substantial diameter. At the lower ends of the columns there are downwardly extending legs of reduced diameter. The downwardly extending legs are connected together by diagonal bracing members. The substructure is held in place by a conventional spread of (catenary) anchor chains.
In the substructures illustrated in these two patent specifications, the downwardly extending legs have diagonal truss or bracing members between them. Moreover, in both cases there are sudden transitions of cross section (from the hull or corner columns respectively, to the legs of reduced diameter).
The substructures in the patent specifications referred to above are designed to be built in a horizontal attitude. The truss or bracing members require significant fabrication activity. Fit up and welding of these members adds cost and takes up time in the fabrication schedule. When complete, the substructures can be floated out (still in a horizontal attitude) to their required locations. At those locations they have to be upended. The sudden transitions of cross section may create instability during floatout and upending.
Multi leg substructures have smaller diameter columns, and so need less reinforcement to resist hydrostatic crushing loads, than do the hulls of spars. This results in lighter substructures to support similar topside weights. Additionally, the spacing apart of the columns gives better stability characteristics.
The present invention is intended to take advantage of these characteristics of a multi leg configuration, while avoiding the need for complex truss or bracing members, extending between the columns. It is also intended to reduce the disadvantages of discontinuities in cross section in the columns.